CORE LAB ONLY - SAMPLE PROCESSING. Recent advances in our understanding of the dynamic nature of HIV infection and of the importance of HIV-1 replication in disease pathogenesis, coupled with the advent of new potent antiretroviral agents such as the protease inhibitors, have led to the development of new treatment paradigms. For most patients, three-drug regimens including two reverse transcriptase (RT) inhibitors and a protease inhibitor are needed to reduce and maintain plasma levels of HIV-1 RNA below the limits of detection. Despite the success of these regimens, treatment failures may occur for a variety of reasons. This study addresses the question of whether to discontinue lamivudine or substitute the non-nucleoside RT inhibitor delavirdine when adding indinavir to patients previously treated with didanosine, or d4T plus 3TC who have >500 copies/ml of plasma HIV-1 RNA.